As a modulation system of large-capacity signal transmission, a polarization-multiplexed digital coherent transmission system is promising, in which X-polarization light and Y-polarization light are each modulated and then polarization-multiplexed, and demodulated by a digital coherent receiver. Here, the coherent transmission system is a modulation system, in which a phase of light or even an amplitude in addition to the phase of light is modulated on the transmission side and on the reception side, local oscillation light (LO light) and signal light after transmission are mixed using an interference circuit, called a 90-degree hybrid, and received by a balanced photo detector (B-PD), and thereby, the signal light is demodulated by splitting the signal light into the real number part and the imaginary number part when the electric field of light is regarded as a complex number. However, in the receiver, the input polarization-multiplexed signal is polarization-split optically, but in general, the base polarization state of a polarization beam splitter (PBS) used therein and the base polarization state of the polarization-multiplexed signal light do not coincide with each other, and therefore, two orthogonal polarization components output from the PBS will not form the signal light the polarization multiplexing of which is demultiplexed.
However, by performing digital signal processing on an electric signal output from the B-PD as to the respective components polarization-multiplexed optically, it is possible to perform polarization demultiplexing. Further, by the digital signal processing, it is also possible to estimate the relative phase difference between the signal light and LO light and to perform processing, such as dispersion compensation and error correction.
As described above, the system that considerably simplifies optical processing by performing digital signal processing on a polarization-multiplexed coherent modulated signal in the receiver and further improves reception characteristics also is called a polarization-multiplexed digital coherent transmission system and very promising.
As a typical and practical method of the coherent modulation system, the spread of a quadrature phase shift keying is being encouraged and the quadrature phase shift keying by polarization multiplexing is known as a DP-QPSK (Dual Polarization Quadrature Phase Shift Keying). With the DP-QPSK modulation system, when the symbol rate is 10 GSymbol/s, the bit rate is 40 Gbit/s and when the symbol rate is 25 GSymbol/s, the bit rate is 100 Gbit/s, and therefore, it is possible to improve frequency use efficiency. The DP-QPSK modulation system when simply referred to means a system that applies a digital coherent receiver at the time of demodulation.
In the DP-QPSK modulation system demodulator (DP-QPSK demodulator), first, a DP-QPSK signal, which is a multiplexed signal of an X-polarization QPSK signal and a Y-polarization QPSK signal, is split into an X-polarization QPSK signal and a Y-polarization QPSK signal by a polarization beam splitter (PBS). Further, by an X-polarization 90-degree hybrid and a Y-polarization 90-degree hybrid, the split X-polarization QPSK signal and Y-polarization QPSK signal, and local oscillation light (LO light) are mixed, respectively. By receiving this mixed light with a B-PD combined together, each polarization phase-modulated signal (QPSK signal light) is converted into an intensity-modulated signal and components corresponding to the real part and the imaginary part (I-component and Q-component) of the electric field of the signal light in each polarization are extracted independently. In general, the 90-degree hybrid is known as a circuit that branches the input signal light and local oscillation light into two, respectively, gives a phase difference of 90 degrees to the local oscillation light branched into two as a relative phase difference of lightwave, and then mixes one of the signal light branched into two and one of the local oscillation light into two, and the other of the signal light branched into two and the other of the local oscillation light into two, respectively.
The PBS and the 90-degree hybrid are realized individually by a space optical system or quartz-based planar lightwave circuit (PLC) as prior art (see Non-Patent Documents 1 to 6).
In Non-Patent Documents 1 to 3, the 90-degree hybrid having a configuration in which a coupler and a PBS are combined on one PLC is disclosed and the technique to reduce the time difference (skew) between I- and Q-components by making the same the optical waveguide lengths between the I- and Q-components, respectively is disclosed.
In Documents 4, 5, 6, the 90-degree hybrid having a configuration in which a PBS is formed on one PLC is disclosed. In Document 6, the PLC on which a plurality of PBS's is cascade-connected and which is formed into a two-stage configuration is disclosed.
[Non-Patent Document 1] Y. Inoue et al., “Optical 90-degree Hybrid using Quartz-based PLC” 1994 Autumn IEICE Conference, C-259
[Non-Patent Document 2] M. Hosoya et al., “Construction Technology of 90° Hybrid Balanced Optical Receiver Module Using PLC” IEICE Technical Research Report, Optical Communication System OCS-95 pp. 49-54
[Non-Patent Document 3] S. Norimatsu et al., “An Optical 90-Hybrid Balanced Receiver Module Using a Planar Lightwave Circuit,” IEEE Photon. Technol. Lett., Vol. 6, No. 6, pp. 737-740 (1994)
[Non-Patent Document 4] M. Okuno et al., “Birefringence Control of Silica Waveguides on Si and Its Application to a Polarization-Beam Splitter/Switch,” J. Lightwave Technol., Vol. 12, No. 4, pp. 625-633
[Non-Patent Document 5] Y. Hashizume et al., “Integrated polarisation beam splitter using waveguide birefringence dependence on waveguide core width,” Electron. Lett., vol. 37, No. 25, pp. 1517-1518 (2001)
[Non-Patent Document 6] N. Matsubara et al., “SILICA-BASED PLC-TYPE POLARIZATION BEAM SPLITTER WITH >30 dB HIGH EXTINCTION RATIO OVER 75 nm BAND WIDTH,” MOC2005, C2 (2005)